1. Field of the Invention
The present invention relates to a fixing device which is mounted in an image forming apparatus such as a copying machine or printer and fixes a developer image on a paper sheet.
2. Description of the Related Art
Conventionally, an image forming apparatus such as an electrophotographic copying machine utilizing a digital technique comprises a fixing device which fixes a developer image onto a paper sheet by heating in a press state.
The heat source of the heating roller of the fixing device is induction heating. In induction heating, a coil is stored in the heating roller and connected to a capacitance to form a resonant circuit. One resonant circuit is excited at one frequency. A high-frequency current is supplied to the coil to generate a high-frequency magnetic field from the coil. The high-frequency magnetic field causes the heating roller to generate an eddy current. Joule heat by the eddy current causes self-heating of the heating roller.
In recent years, a short warming-up time presents a technical issue as an energy-saving technique. The measure is to decrease the diameter of the heating roller.
The electrophotographic copying machine uses various paper sizes. When small-width paper sheets are successively fed to the fixing device, a portion of the heating roller extending from the paper becomes higher in temperature than a portion within the paper width because it is not deprived of heat by the paper sheet. When a large-width paper sheet is fed after a small-width paper sheet, a fixing error occurs due to a high-temperature offset. This phenomenon is prominent for a smaller-diameter heating roller (smaller heat capacity).